Secondary glaucoma in uveitis patients is most often open angle glaucoma, but closed angle glaucoma is also seen.
Open angle glaucoma is glaucoma in which the aqueous can still pass out of the eye, however, it drains out too slowly and as the fluid builds up, pressure rises.
Closed angle glaucoma occurs when the aqueous cannot flow out because the iris has blocked the trabecular meshwork. This type of glaucoma is often painful and immediate treatment is needed to prevent permanent loss of vision.
The Diagnosis of Glaucoma
Intraocular pressure is measured through tonometry – if you’re the one on the other side of the slit lamp, it’s the blue light test.
A high intraocular pressure does not mean an optic nerve is damaged, but if pressure remains high for a sustained period, or if there are persistent changes beyond the normal diurnal changes in pressure, then glaucoma will be suspected.
Conversely, some glaucoma patients have normal tension glaucoma in which the intraocular pressure lies within the normal parameters. The doctor may also be interested in corneal thickness since patients with thin corneas may have artificially low pressures and those with thicker corneas may have seemingly high intraocular pressure. Tests other than just the measurement of intraocular pressure are needed before glaucoma is diagnosed.
The slit lamp exam enables the doctor to see the optic nerve and changes in its appearance will be noted. The optic nerve looks like a cup and when glaucoma damage begins, the cup looks larger. For more details, see a diagram showing cupping and photos of optic nerves.
There are further tests which will show if the optic nerve has suffered damage from glaucoma. Two of these are the Heidelberg Retinal Tomography test (HRT) and the Optical Coherence Tomography test (OCT). It is common to have these tests carried out annually but more frequently if slit lamp exam suggests further change in optic cupping or a visual field test shows loss of visual field. Neither of these tests hurt and all that is required of the patient is to sit still and keep a steady gaze.
The HRT test scans the retinal surface and the optic nerve with a laser and measures the thickness of the nerve fibres. (The more fibres, the smaller the cup).
This test will chart suspect areas where damage to the fibres is possible, as well as showing healthy areas and areas where fibres have died.
The OCT test creates a contour map of the optic nerve and the optic cup and measures the retinal nerve fibre thickness.
Other tests detect loss of visual field and require much more from the patient in terms of concentration and keeping gaze steady.
This is a computerised test and you sit in front of a screen, are given a clicker and click each time you see a small light flash in your side vision. Some of these lights will be brighter than others. You are supposed to keep looking at the centre of the screen and not let your eye wander looking for the next light.
This is a similar test which follows the same procedure, except some poor technician or doctor is behind the screen controlling the flashing lights.
A third test is one for driving and is carried out by instruction of the DVLA. It is quite common to think you are going mad doing these tests, or to become clicker happy. Your doctor will probably have an OCT result to hand and will be able to interpret your visual field test, even if you’ve clicked for flashing lights that are not actually there.
The Good Hope Eye Department has information and examples of visual field testing.